Here is my theory: If we are to believe that the Anime Pokemon Universe and the Video Game Pokemon Universe are the same then when Ho-oh flies over Ash in episode 1 the save file that Ash was using got corrupted and his character data was corrupted with it because Ho-oh isn`t supposed to be in the game. therefore he is forever 10 and occasionally suffers from massive brain damage. just like missing no. JKJK